Application of SVE to Treatment of Oxygenates

During SVE, contaminated soil vapors are extracted by inducing a vacuum at one or more extraction points that are typically constructed as vertical vapor extraction wells. Horizontal extraction wells or trenches have also been employed as extraction points. In general, SVE is applied at depths

300 250 200

1 150

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FIGURE 24.4 Ranges of vapor pressures for common fuel oxygenates. (Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04-009, United States Environmental Protection Agency, Washington, DC, May 2004.)

FIGURE 24.4 Ranges of vapor pressures for common fuel oxygenates. (Adapted from U.S. EPA, Technologies for Treating MTBE and Other Fuel Oxygenates, EPA 542-R-04-009, United States Environmental Protection Agency, Washington, DC, May 2004.)

MTBE ETBE TAME DIPE TBA Ethanol Methanol Benzene Toluene Ethybenzene Xylenes

MTBE ETBE TAME DIPE TBA Ethanol Methanol Benzene Toluene Ethybenzene Xylenes ranging from 10 to 50 ft below ground surface, but has been applied as deep as 300 ft.40 Shallower applications typically employ some manner of a surface seal to minimize short-circuiting of the system by ambient air. Typical flow rates for extracted soil vapors range from 60 to 700 ft3/min. The vacuum pressures required at the top of the vapor extraction well (wellhead vacuum) to produce the desired vapor extraction rate typically range from 3 to 100 in. of water, and vary depending on soil permeability.34,40 The ROI of an extraction well is used to determine the number and spacing of extraction wells. The ROI is the distance from an extraction well to the point at which a vacuum can be induced to enhance volatilization and extraction of contaminants from the soil.

SVE is considered to be most effective in more homogeneous and higher-permeability geologies because subsurface preferential pathways may result in short circuiting and tighter formations may minimize the ROI of extraction points. In general, vapor extraction points are designed and spaced to provide for a reduced pressure gradient throughout the contaminated zone. Remediation of MTBE and other oxygenates using SVE may also potentially benefit from the aerobic conditions generated by subsurface air flow that may result in conditions that are amenable to in situ biodegradation of contaminants. At some sites, groundwater or free product extraction equipment may be incorporated into vapor extraction points.

Extracted soil vapors, containing volatilized contaminants, are routed to an aboveground treatment system prior to being discharged to the atmosphere. The types of aboveground vapor treatment technologies that have been used for treating MTBE and other fuel oxygenates include the following39:

1. Adsorption—Processes in which vapor-phase contaminants are adsorbed onto a medium, such as GAC or resin, as driven by equilibrium forces.

2. Thermal treatment—Processes in which vapor-phase contaminants are destroyed via high-temperature oxidation; the primary categories of thermal treatment used to treat MTBE and other oxygenates include thermal oxidation, which employs a flame to generate the high temperatures needed to oxidize contaminants, and catalytic oxidation, which employs lower temperatures in the presence of a catalyst (typically platinum, palladium, or other metal oxides) to destroy contaminants.

3. Biofilters—Processes in which contaminants are biodegraded in a fixed-film bioreactor, typically consisting of a bed of high surface area filter media, such as GAC, that acts as a support matrix for a thin film consisting of microbes that are acclimated to the biodegradation of MTBE or other contaminants.

The type of vapor treatment that is used will depend on factors such as the contaminant concentrations in the extracted vapors and the air emission discharge limitations for the site. Highly contaminated vapors at a site with stringent air emission limitations may require a multistep vapor treatment train, such as thermal oxidation, followed by carbon adsorption. Less contaminated vapors at a site with less stringent air emission limitation may require minimal or no vapor treatment. Fields et al.38 described the following rules of thumb for selecting vapor treatment:

• Thermal oxidation for VOC concentrations greater than 2000 ppmv.

• Catalytic oxidation for VOC concentrations between 100 and 2000 ppmv.

• GAC treatment for VOC concentrations between 35 and 100 ppmv.

• Direct discharge for VOC concentrations <35 ppmv.

More detailed information on the application of SVE at sites contaminated with MTBE and other oxygenates in general is available in Refs [26,39,45].

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